Pneumocystis (Pc) is a major cause of lethal pneumonia (P) in HIV patients, but seroepidemiological studies have been severely limited by the lack of suitable reagents. I have developed recombinant fragments (A, B,C) of the human PC major surface glycoprotein (Msg) and found that one of them, MsgC, shows considerable promise as an antigen for these studies in an enzyme linked immunosorbent assay (ELISA). I hypothesize that MsgC is a universal serologic marker for PC infection, and that the highest level of antibodies will be found in HIV patients who recovered from PcP (PcP+). The goals of this proposal are to analyze serum antibody responses, as well as the host factors that influence these responses, to the Msg fragments in adult HIV PcP+ patients, HIV patients who never had PcP (PcP-), and healthy blood donors in 8 different international locations. Data obtained from these studies will lead to the first standard antigen for PC serologic studies, and enhance our understanding of the epidemiology and immunology of PC infection.